Thermionically inactive electrode



July 5, 1932. J. 1.. WUERTZ. 1,865,449

THERMIONICALLY INACTIVE ELECTRODE Filed Sept. 28, 1926 john L .Wuercz 'FG 2 INVENTOR ATTORNEY Patented July 5, 1932 UNITED STATES PATENT OFFICE v JOHN L. WUERTZ, OF BLOO'MFIELD, NEW JERSEY, ASSIGNOR TO WESTINGHOUSE m COMPANY, A CORPORATION OF PENNSYLVANIA THERMIONICALLY mao'rrvn maao'rnonn Application filed September 28, 1926. Serial No. 138,170.

This invention relates to an electron discharge device employing, as the-active electron-emitting element, a cathode coated with the oxides of the alkaline earth-metals and 6 more particularly it relates to a grid or an anode for such a device.

Considerable difficulty has been experienced heretofore in the manufacture of electron discharge devices employing oxide-coated cath- 10 odes due to electron emission from the grid or the plate, caused by the deposition on the plate and grid electrodes of the alkaline earth oxides from the cathode coating. This trouble is particularly objectionable in electron dis- 1 charge devices which operate at high voltages whereby the grid or plate become heated during the operation. For instance, in high wattage rectifiers employing oxide-coated cathodes, the anode attains a sufficient temperature to show a perceptible dull red color and the electron emission from such electrode is suflicient to heavily bombard the cathode and quickly disintegrate the same, thus rendering the device short lived. This diflicu'lty is of 95 sufiicient importance to preclude or greatly retard the use of oxide-coated cathodes for high voltage devices.

One of the objects of the present invention is to provide an oxide-coated cathode electron discharge device which may be operated at high voltages and in which the electron emission from the grid and anode will be negligible in amount.

Another object is to provide a grid or anode for an electron discharge device of the oxidecoated type which will counteract the electron-emitting effect of the, alkaline earth oxides deposited thereon from the cathode.

Other objects and advantages will appear hereinafter.

In accordance with the present invention, the grid or anode of an electron discharge device employing a cathode coated with the oxides of the alkaline earth metals is coated with a thin layer of a substance which, under suit able conditions, combines with the alkaline earth oxides to reduce the oxide to the metal. A material which I have found particularly suitable for this purpose is graphite. The 59 graphite may be mixed with any suitable binder andapplied to the electrodes in any desired manner and dried thereon. Preferably, I use a suspension of graphite and water, such as aquadag of such consistency that it may be readily applied to the electrodes by spray ng, although the coating may be accomplished by painting, dipping or any other convenient manner.

Qare should be taken to coat the electrodes entlrel-y with the reducing material since if anyof the metal of the electrode is exposed 1t will permit the alkaline earth oxides upon deposition thereon, to emit electrons with the detrimental effects mentioned above. In spraying the aquadag upon the grid or plate electrodes they are preferably heated to around 150 to 200 C. and maintained at this temperature for a short period after the spraying is completed to thoroughly dry the aquadag thereon. I have found that if the aquadag is baked onto the electrodes for about three minutes at the tem erature specified, an adherent coating will e obtained. For convenience in spraying the electrodes the coating is preferably applied thereto before they are mounted.

In order that the invention may be more fully understood, reference will be had to the accompanying drawing in which:

Fig. 1 is an elevation partly in section of an oscillator tube employing an oxide-coated cathode and having the grid electrode coated wish a reducing material such as graphite; an

Fig. 2 is an elevation partly in section of the mount on a rectifier tube having the anode similarly coated.

The device shown in Fig. 1 comprises an envelope 1 having a cathode 2, grid 3 and anode 4, mounted on suitable support wires 5 sealed in the press 6 of the device. The electrodes are connected to leading-in conductors 7 joined to contact pins 8 carried by the base 9.

A metallic clean-up agent 10, such as mag-' nesium or misch metal, is attached to the plate electrode 4 and is vaporized, as by high frequency induction heating, to clean-up residual gases and form an active deposit on the wall of the envelope for maintaining the device in a hard condition.

The cathode 2 consists of any suitable metallic filamentary base having a coatin of the oxides of the alkaline earth metals t ereon. The grid or control electrode '3 has applied thereto an adherent layer of a reducing material such as carbon or graphite which is substantially non-volatile at the operating temperature of the electrode and which ,has substantiall no electron emission at that temper ature. pon initial operation of the device a small amount of the oxide coating from the cathode is sputtered, vaporized or otherwise carried over to the control electrode and deposited thereon. In orderto render this deposit thermionically inactive, the grid is heat ed to a red heat, at which temperature, and under the vacuous conditions in the device, a reaction appears to take place, probably resulting in 'the'reduction of the oxides of the alkaline earth metals by the carbon producing free alkaline earth metal whic va orizes away and an oxide of carbon whic is quickly cleaned up by the metallic getter 10 contained in the envelope. Whatever the true theory, the graphite effectively prevents the formation of active deposits of the alkaline earth metals on the grid electrode. The grid may be heated in any desired manner but I prefer to impress a potential between the filament and the grid to heat the latter by electron bombardment.

If desired, the plate electrode or anode 4 may be similarly coated. However, except when employed for rectification purposes I have found this unnecessary.

In Fig. 2 the rectifier is shown comprising an oxide-coated cathode 11 and a complimentary electrode or anode 12, the latter of which is coated with a layer of graphite'or aquadag for suppressing the electron emission due to the alkaline earth oxides deposited thereon from the cathode. In this embodiment the plate is conveniently heated by high frequency induction.

While I have shown a preferred embodiment of my invention it is to be understood that various modifications and changes may be made therein without departing from the invention and I do not deslre to be limited except by the appended claims.

What is claimed is:

1. An electron discharge device comprising an evacuated envelope containing a cathode coated with the oxides of the alkaline earth metals and a second electrode entirely coated with carbon, non-volatile at the operating temperature of the electrode and capable of combining with the material of the cathode on coming in contact therewith to render the same thermionically inactive and a clean-up agent within the envelope for fixing the reaction products of said combination.

2. The method of preventing secondary electron emission from an electrode of an electron discharge device having a cathode coated esa r49 with an oxide of an alkaline earth metal which includes entirely coating the electrode with carbon, incandescing the cathode, and thereafter heating the electrode to a temperature suficient to effect a reaction between the carbon and any alkaline earth oxide thereon.

J GEN L. WUERTZ. 

